Why does wrapping os_log() cause doubles to not be logged correctly?

Question

Consider the following example:

import Foundation
import os.log

class OSLogWrapper {

    func logDefault(_ message: StaticString, _ args: CVarArg...) {
                


        

Answer 1

As mentioned in my comment to Rob Mayoff's answer above, for anybody experiencing the same kind of issue with os_signpost(), here is a wrapper class I made around it:

import Foundation
import os
import _SwiftOSOverlayShims

public final class Signpost {

    private final let log: OSLog

    public init(log: OSLog) {
        self.log = log
    }

    public final func begin(name: StaticString, dso: UnsafeRawPointer = #dsohandle, idObject: AnyObject? = nil) {
        if #available(iOS 12.0, *) {
            signpost(.begin, dso: dso, name: name, idObject: idObject)
        }
    }

    public final func begin(name: StaticString, dso: UnsafeRawPointer = #dsohandle, idObject: AnyObject? = nil, _ format: StaticString, _ arguments: CVarArg...) {
        if #available(iOS 12.0, *) {
            signpost(.begin, dso: dso, name: name, idObject: idObject, format, arguments)
        }
    }

    public final func event(name: StaticString, dso: UnsafeRawPointer = #dsohandle, idObject: AnyObject? = nil) {
        if #available(iOS 12.0, *) {
            signpost(.event, dso: dso, name: name, idObject: idObject)
        }
    }

    public final func event(name: StaticString, dso: UnsafeRawPointer = #dsohandle, idObject: AnyObject? = nil, _ format: StaticString, _ arguments: CVarArg...) {
        if #available(iOS 12.0, *) {
            signpost(.event, dso: dso, name: name, idObject: idObject, format, arguments)
        }
    }

    public final func end(name: StaticString, dso: UnsafeRawPointer = #dsohandle, idObject: AnyObject? = nil) {
        if #available(iOS 12.0, *) {
            signpost(.end, dso: dso, name: name, idObject: idObject)
        }
    }

    public final func end(name: StaticString, dso: UnsafeRawPointer = #dsohandle, idObject: AnyObject? = nil, _ format: StaticString, _ arguments: CVarArg...) {
        if #available(iOS 12.0, *) {
            signpost(.end, dso: dso, name: name, idObject: idObject, format, arguments)
        }
    }

    @available(iOS 12.0, *)
    private final func signpost(_ type: OSSignpostType, dso: UnsafeRawPointer = #dsohandle, name: StaticString, idObject: AnyObject? = nil) {
        guard log.signpostsEnabled else { return }
        let signpostID = getSignpostId(forObject: idObject)
        os_signpost(type, dso: dso, log: log, name: name, signpostID: signpostID)
    }

    @available(iOS 12.0, *)
    private final func signpost(
        _ type: OSSignpostType,
        dso: UnsafeRawPointer,
        name: StaticString,
        idObject: AnyObject? = nil,
        _ format: StaticString,
        _ arguments: [CVarArg])
    {
        // This crazy mess is because [CVarArg] gets treated as a single CVarArg and repassing a CVarArg... actually passes a [CVarArg]
        // This was copied from the publicly available Swift source code at https://github.com/apple/swift/blob/master/stdlib/public/Darwin/os/os_signpost.swift#L40
        // THIS IS A HACK
        guard log.signpostsEnabled else { return }
        let signpostID = getSignpostId(forObject: idObject)
        guard signpostID != .invalid && signpostID != .null else { return }
        let ra = _swift_os_log_return_address()
        name.withUTF8Buffer { (nameBuf: UnsafeBufferPointer) in
            // Since dladdr is in libc, it is safe to unsafeBitCast
            // the cstring argument type.
            nameBuf.baseAddress!.withMemoryRebound(to: CChar.self, capacity: nameBuf.count) { nameStr in
                format.withUTF8Buffer { (formatBuf: UnsafeBufferPointer) in
                    // Since dladdr is in libc, it is safe to unsafeBitCast
                    // the cstring argument type.
                    formatBuf.baseAddress!.withMemoryRebound(to: CChar.self, capacity: formatBuf.count) { formatStr in
                        withVaList(arguments) { valist in
                            _swift_os_signpost_with_format(dso, ra, log, type, nameStr, signpostID.rawValue, formatStr, valist)
                        }
                    }
                }
            }
        }
    }

    @available(iOS 12.0, *)
    private final func getSignpostId(forObject idObject: AnyObject?) -> OSSignpostID {
        if let idObject = idObject {
            return OSSignpostID(log: log, object: idObject)
        }
        return .exclusive
    }
}